One type of pump for pumping molten metal rotates an impeller on the end of a shaft inside an impeller chamber of a base submerged in the molten metal. The impeller chamber may include a volute or may be a nonvolute chamber as disclosed in U.S. Pat. No. 5,203,681. The pump may be used to circulate molten metal or to transfer molten metal. In a circulation pump, a discharge passageway extends from the impeller chamber to an exterior surface of the base. In a transfer pump, the base includes a socket that receives a riser or conduit that extends to another location where it is desired to move the molten metal. The discharge passageway extends from the impeller chamber to the socket.
It has been proposed to introduce reactive and/or inert gas into molten metal for various metallurgical and processing purposes at different locations in and around the pump. For example, gas has been fed into the molten metal along a conduit at a pump discharge passageway downstream of an impeller chamber. However, this design does not efficiently mix the gas and molten metal and unmixed gas can be liberated from the molten metal bath, which wastes gas and can cause safety problems in the case of toxic chlorine gas. Gas has also been introduced along a passageway in a motor driven shaft for release from an impeller mounted to the end of the shaft, inside the impeller chamber. However, this requires effective seals in view of the travel of gas along a rotating shaft. In addition, introducing gas directly into the impeller chamber, while efficiently mixing the gas and molten metal, can cause cavitation that can lead to serious wear or destruction of pump components including the shaft and impeller. Thus, conventional feeding of gas to a pump for molten metal is impractical or inefficient. Gas has also been injected along a lance supported in the molten metal upstream of an inlet protector.
A pump for pumping molten metal proposed in U.S. Pat. No. 4,786,230 by the inventor included two discharge passages extending from an outlet passage. The outlet passage communicates with two impeller chambers. Two impellers were proposed to be mounted on a motor driven shaft for simultaneous rotation in the impeller chambers. The gas was proposed for use in very small quantities for changing the direction of the molten metal flow, not for treating molten metal with gas.